Attempts to illuminate an object embedded in or hidden by a light-scattering medium are often difficult to accomplish reasonably. For example, while X-rays may be employed to image bones, the high energy of X-rays limits their usefulness due to potential damage to tissue. Also, in the detection of breast cancer in women, present mammography requires exposure of breast tissue to X-rays; this procedure, repeated several times over the course of a woman's man's life, has its own risks of creating cancer, even though the benefits may outweigh the risks.
As a general rule, light, though of a lower energy and thus not as damaging to tissue, cannot be used to image such objects. This is due to the fact that most of the light going through a light-scattering medium, such as flesh, is scattered; only a small fraction of the incident light is not scattered.
Suppose one wanted to view a light-absorbing object embedded in or behind a light-scattering medium. If it were possible to see an image of such an object, the image would appear as a shadow-gram of the object. However, in general, the light scattered by the medium fills in the shadow of the object produced by the unscattered light, thus making the object invisible.
A key fact is that the portion of light that is not scattered emerges first from the light-scattering medium, while the scattered light emerges later. One could use this fact to produce an image of the object, if one could somehow provide a shutter to separate the light that is scattered from that which is not. Note, however, that the time resolution of such a shutter, which depends on the sample thickness, must be on the order of 10.sup.-12 seconds, or 1 pico-second (ps). Such a requirement eliminates mechanical shuttering, which cannot physically respond in such short times.
One approach provided by the prior art is to use a Kerr shutter. Such a device incorporates a cell of a liquid, such as CS.sub.2, positioned between two polarizers, set orthogonal to each other so that light incident on one polarizer does not pass through the other. When an intense beam of light is directed onto the liquid cell, the intense beam disturbs the polarization of light passing through the cell, so that light incident on the one polarizer can now pass through the other polarizer. When the intense beam is turned off, the liquid relaxes back to its original state, and the light is again blocked by the pair of polarizers. The typical relaxation time for this method is about 1 ps.
Another approach in the prior art is known as "light-in-flight" (LIF) recording by holography, developed by N. Abramson and co-workers. In this method, a short coherence length pulse is split into two parts and geometrically ar
ranged to both illuminate an object and scan a reference beam across a holographic plate so that holographic images can be formed in a continuous time sequence across the plate. A related approach, called "chrono-coherent imaging" (CCI) is an adaptation of LIF recording to medical imaging. Unlike LIF recording, which is done in air, medical imaging applications have a liquid medium and do not emphasize reflectivity of solid objects. The imaging modality is differential reflectivity or absorption, and it involves the effects of shadows from differential absorption (or differential scattering). In both cases, wavefronts interfere in successive times to produce a hologram that, when viewed along its length, recreates the time sequence.
Like the Kerr shutter, the LIF and CCI techniques also isolate the first-arriving light, and provide jitter-free, low-power, high-pulse-repetition-rate, two-dimensional imagery, with a temporal resolution virtually the same as the pulse duration. However, the LIF-type configuration does not necessarily produce the best spatial resolution, especially with short pulse durations in the femtosecond domain.
A variation of the holographic recording method is to replace the photographic recording plate by a high-resolution electronic camera. The camera records the hologram, which can then be sent to a computer for analysis; see, e.g., Chen et al, Optics Letters, Vol. 16, No. 7, pp. 487-489 (1991).
Another method for isolating the first-arriving light is to use the process of second-harmonic generation; see, e.g., Yoo et al, Optics Letters, Vol. 16, No. 13, pp. 1019-1021 (1991). In this method, a short pumping pulse of frequency .omega. and photons from the scattering medium (also at frequency .omega.) are made to overlap in a frequency-doubling crystal. If photons from the two light sources reach the crystal at the same instant, then the crystal will emit a photon of frequency 2.omega. in a particular direction. This frequency-doubled light is easily detected. (If the photons do not reach the crystal at the same time, then, in principal, no frequency-doubled light is observed.) The pump pulse is adjusted to arrive at the crystal at the same time as the unscattered light from the scattering medium, so that the amount of frequency-doubled light is proportional to the amount of unscattered light reaching the crystal. The sample is scanned, point by point, and the amount of frequency-doubled light is recorded for each point in the sample. In this way, a two-dimensional image of the unscattered light is laboriously constructed. A disadvantage of this scheme is that the entire image cannot be acquired at the same time; instead, the sample must be slowly scanned, point by point.
A need remains for a technique for storing and producing images of an object obscured by a light-scattering medium, using light.